In a glass tube, detector tubes contain a chemical preparation that reacts with the substance to be detected while undergoing a change in color, while the conversion of the substance is quantitatively displayed in the form of a color length display. For sampling, the gas tube is first opened by breaking off the tips of the detector tube on both sides. The detector tube is then placed into a sampling pump in order to draw a predetermined volume of the test gas through the detector tube. A sampling pump, with which a gas sample can be delivered through a detector tube stroke by stroke, has been known from DE 38 22 001.
However, the prior-art device for sampling gas cannot be used if very small amounts of substances that are also present in the ambient air atmosphere are to be detected. For example, a detector tube that is highly sensitive to water vapor must be used to detect water vapor in natural gas. If the tips of the detector tube are broken off only before the beginning of the measurement and the detector tube is placed into the sampling pump, the substance to be detected, which is present in the detector tube, is preloaded by the water vapor entering from the ambient atmosphere via the open tips of the detector tube such that measurement is no longer possible.